Hartley oscillators are commonly used in the field as a means of generating stable oscillator signals. In general, a Hartley oscillator includes a feedback circuit formed via a tapped inductor. In the prior art, this inductor is implemented either with discrete components or with micro-strip transmission lines. The latter is realized by printing a transmission line on a substrate providing for an easy realization of the tapped line. In oscillator with tapped micro-strip transmission line the tapped inductor normally serves as the main resonator. In such applications, the Q of the tank circuit is determined by the combination of the varactor network, the tuning capacitor and the tapped inductor.
One of the major advantages of a Hartley configuration is the minimization of capacitance in the tank circuit. This is accomplished by including the tapped inductor in the feedback network. The bandwidth of the oscillator for a constant tuning range may be increased simply by lowering the capacitance of the tank circuit. A problem that is encountered is the sideband noise that is increased as the bandwidth is increased. Attempts to lower this effect include increasing the Q and power both of which are generally costly to attain.
It is therefore desired to have an oscillator with maximum bandwidth and minimum sideband noise without significant effect on the oscillator Q.